Thermal protection systems are employed in a wide variety of applications including, but not limited to, interior surfaces of jet and rocket engines and exterior surfaces of vehicles for protection against hot, convective flow passing over the vehicle. For example, a thermal protection system (TPS) may be applied to hypersonic vehicles and reusable launch vehicles to provide a thermal shield against extreme temperatures to which the vehicle is subjected. As applied to reusable launch vehicles, a TPS must be capable of protecting the vehicle substructure against temperature extremes ranging from −300° F. on orbit to 3000° F. during re-entry into the Earth's atmosphere. In this regard, a TPS must be capable of maintaining the temperature of the vehicle's metallic and/or composite substructure below the temperatures at which the mechanical properties of the substructure begin to degrade.
As applied to vehicles such as the Space Shuttle, a TPS may comprise a large number of insulative elements that may be mounted on the substructure for protection against high-temperature convective flow. For example, the TPS may comprise a plurality of ceramic foam tiles which may be configured as passive thermal tiles or as actively-cooled tiles. Actively-cooled tiles may include channels through which coolant may be circulated in order to draw excess heat from the tiles and/or substructure.
In addition to protecting substructure against temperature extremes, a TPS must also accommodate relative movement of the substructure under static and dynamic loading conditions. For example, a TPS must be capable of accommodating flight-induced deflections of a vehicle substructure. Tile gaps may be provided between tiles in order to accommodate such relative movement of the substructure. In addition, tile gaps may be provided between tiles to accommodate differences in the thermal expansion properties of the tiles relative to the thermal expansion properties of the substructure to which the tiles are mounted. For example, during orbital maneuvers of a reusable launch vehicle, temperatures can vary by several hundred degrees Fahrenheit causing differences in thermal expansion of the tiles relative to the airframe substructure. The tile gaps must be sized in order to accommodate such differences in thermal growth.
However, tile gaps must also be sealed to minimize heating of the substructure at the bottom of the tile gap by high temperature convective flow passing over the tile exterior surfaces. In addition, it is desirable to seal the tile gaps in order to maintain the aerodynamics or continuity of flow over the tile exterior surfaces which may comprise an outer mold line of a vehicle. In this regard, the tile gaps must be sealed to minimize aerodynamic pressure losses otherwise associated with open tile gaps. Due to the relatively large number of thermal tiles employed in a given application (e.g., tens of thousands on a single Space Shuttle), it is desirable that seals for the tile gaps are relatively easy to install and are securely maintained in position within the tile gap to prevent extraction by high speed flow passing over the tile exterior surfaces.
Included in the prior art are several seal assembly configurations for sealing tile gaps. One prior art configuration comprises thermal padding or filler bar which may be force-fitted into the tile gaps. Following force-fitting into the tile gaps, the thermal padding or filler bar may be coated with a high-temperature hardening compound to provide a smooth surface over the tile gap that is continuous with the tile exterior surfaces. Another prior art seal assembly configuration includes the use of adhesives for bonding insulating materials to tile side surfaces of opposing thermal tiles.
Another prior art sealing assembly configuration includes the use of flaps that are mechanically secured in position by sandwiching the flaps between the thermal tiles and the substructure. A further prior art configuration includes the use of an insulating material which is clamped in position beneath a bracket mounted to the substructure. The insulating material forms a liner on the opposing tile side surfaces and extends upwardly to the tile exterior surface in order to maintain continuity of the outer mold line across the tile gap.
While the above described prior art sealing assembly configurations are generally suitable for their intended purposes, they possess several drawbacks which detract from their overall utility. For example, the hardening compound that is applied over filler bar complicates removal, repair and/or replacement of the tiles. Likewise, seal assemblies which are adhesively bonded within the tile gaps present difficulties in repairing and/or replacing the seal assembly or the tiles to which the seal assembly is bonded. Seal assemblies such as the above-mentioned flaps which are mechanically attached to the tile gaps by sandwiching between the tile and substructure may be costly to manufacture and time-consuming to install.
As can be seen, there exists a need in the art for a seal assembly for sealing tile gaps between tiles which forms a low-thermal conductivity seal to prevent overheating of the underlying substructure. Additionally, there exists a need in the art for a seal assembly which maintains continuity of the outer mold line of the tile exterior surfaces while accommodating differences in thermal expansion of the tiles relative to the substructure. Furthermore, there exists a need in the art for a seal assembly which obviates the need for adhesive bonding or mechanical attachment of the seal assembly to the tile. Finally, there exists a need in the art for a seal assembly for sealing tile gaps which is simple in construction, low in cost and which is easily insertable into the tile gap and securely maintainable in position but which is also easily removable and replaceable in the field.